Materials with a multilayer structure have important applications as optical elements, as they act as interferential filters or Bragg reflectors, capable of selectively reflecting or transmitting a range of electromagnetic frequencies, generally comprised between the ultraviolet and infrared ranges of the spectrum, determined by the thickness and refractive index of the layers. Using more recent terminology, these materials are unidimensional photonic crystals, as they have a periodic modulation of the refractive index in one of the three spatial directions.
The multilayer systems currently available on the market are mostly manufactured using techniques that are normally grouped under the term Physical Vapor Deposition. In all cases, deposition takes place under vacuum conditions, and the solid is condensed directly from the vapor phase. Optical coatings obtained through this kind of technique have great stability against variations in ambient conditions, in addition to high mechanical resistance. There is another large group of multilayer formation methods based on sol-gel type processes. These methods have allowed the development of multilayer coatings, which are highly resistant to damage caused by intense laser radiation, having much higher damage thresholds than other types of structures. However, these multilayer coatings have low mechanical stability, and their properties vary according to the ambient conditions, both phenomena being related to their mesoporosity, due to which they are not suitable as passive optical elements, even though they do have applications in other fields, such as that of sensors. Typically, the pores of a layer grown by sol-gel are irregular in shape, with a very wide size distribution and an average size comprised between 2 nm and 100 nm. A multilayer structure with a controlled mesostructure (shape and size), the optical properties of which can be controlled, would open new application possibilities for these types of materials in different fields. Materials with relatively controlled mesoporosity and which have aroused much interest, although uses for these have not yet been presented, have also been recently developed. They are porous silicon multilayer structures obtained by means of electrochemical dissolution. Very recently, multilayer structures have been developed wherein each layer has an ordered mesoporosity of a finely controlled size, the materials used being silica and titania. This work is the object of a Spanish patent filed in 2006 (application number: 200602405). Finally, there is a reference to the disclosure presented herein in scientific literature that bears close relationship thereto. It relates to the manufacturing of colloidal silica and titania particle multilayers as a reflective or anti-reflective coating carried out by I. M. Thomas in 1987. Although the method described is similar to that presented herein, there is hardly any characterization of the material obtained, due to which it is difficult to know the type of structure that was achieved at the time.
The disclosure presented herein is closely related to these four groups of materials, which are described in further detail below.
Multilayer Materials Obtained by Sol-Gel, Alternating Dense TiO2 and SiO2 Layers
The manufacturing techniques commonly used to synthesize micro-components in solid state are suitable for small areas, of the size of a wafer. If we need to deposit thin laminae on areas of greater size, sol-gel techniques [C. J. Brinker and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic New York, 1990] offer significant advantages: it is a simple method that allows a wide variety of materials (oxides, semiconductors, piezoelectric, ferroelectric, etc.) to be deposited in the form of thin films over different substrates (polymers, ceramics, metals, etc.). The variety of materials that can be deposited allows the design of sol-gel structures in the form of photonic band gap devices or photonic crystals.
Bragg Reflectors or BRs in 1D are the photonic crystals which have achieved greater development due to the sol-gel. Very high reflectivities are obtained in these materials due to the Bragg reflection phenomenon. In general, they are produced by alternating layers of materials with a high and low refractive index, forming a stack of dielectric multilayers. BRs synthesised by sol-gel can be obtained by spin-coating [R. M. Almeida, S. Portal, Photonic band gap structures by sol-gel processing, Current Opinion in Solid State and Materials Science 7 (2003) 151. R. M. Almeida, A. S. Rodrigues, Photonic bandgap materials and structures by sol-gel processing, Journal of Non-Crystalline Solids 326&327 (2003) 405. P. K. Biswas, D. Kundu and D. Ganguli, Preparation of wavelength-selective reflectors by sol-gel processing, J. Mater. Sci. Lett. 6 (1987) 1481] or dip-coating [Chen K. M., Sparks A. W., Luan H. C., Lim D. R., Wada K., Kimerling L. C., SiO2/TiO2, omnidirectional reflector and microcavity resonator via the sol-gel method, Appl. Phys. Lett. 75 (1999) 3805. Hang Q., Li X., Shen J., Wu G., Wang J., Chen L., ZrO2 thin films and ZrO2/SiO2 optical reflection filters deposited by sol-gel method, Mater. Lett. 45 (2000) 311. S. Rabaste, J. Bellessa, A. Brioude, C. Bovier, J. C. Plenet, R. Brenier, O. Marty, J. Mugnier, J. Dumas, Sol-gel manufacturing of thick multilayers applied to Bragg reflectors and microcavities, Thin Solid Films 416 (2002) 242]. The differences between the refractive index values of the materials used and the number of layers are the most important BR parameters. On increasing the difference between the n of the layers and on increasing the number of layers, the greater the reflectivity of the Photonic Band Gap or PBG, forbidden range of wavelengths from the UV to the NIR which are reflected by the dielectric minor. In general, SiO2, TiO2 and ZrO2 are used due to the significant difference between their refractive indices (1.45-1.52, 2.07-2.55, 2.1-2.2, respectively).
The problem with this type of synthesis lies in the fact that, on increasing the number of layers, the risk of fissure development in the material which can damage the structural integrity of the multilayer also increases. In order to solve this problem, Almeida et al. [R. M. Almeida, A. S. Rodrigues, Photonic bandgap materials and structures by sol-gel processing, Journal of Non-Crystalline Solids 326&327 (2003) 405] and Rabaste et al. [Rabaste, J. Bellessa, A. Brioude, C. Bovier, J. C. Plenet, R. Brenier, O. Marty, J. Mugnier, J. Dumas, Sol-gel manufacturing of thick multilayers applied to Bragg reflectors and microcavities, Thin Solid Films 416 (2002) 242] have used very short thermal densification treatments at very high temperatures (1,000° C. for 90 seconds and 900° C. for 2 seconds, respectively), thereby obtaining a stack of up to 60 layers with thicknesses ranging between 80 nm and 100 nm, with a reflectivity of more than 99% (normal incidence). Thermal densification treatments are carried out after the synthesis of each of the layers and, on using such high temperatures, crystallization of the TiO2 of the first layers cannot be avoided, the first layers being subjected to high temperatures for longer time periods due to the reiterated thermal treatments they undergo. Crystal growth must be carefully controlled as it deteriorates multilayer optical quality on introducing Rayleigh dispersion and due to the roughness generated in the interface with the SiO2 layers. Additionally, the first layers undergo a degree of densification different from that of the last layers, which are subject to high temperatures for shorter time periods; this non-homogeneous densification also entails lower multilayer optical quality on modifying optical thickness. [P. K. Biswas, D. Kundu and D. Ganguli, Preparation of wavelength-selective reflectors by sol-gel processing, J. Mater. Sci. Lett. 6 (1987) 1481. Rabaste, J. Bellessa, A. Brioude, C. Bovier, J. C. Plenet, R. Brenier, O. Marty, J. Mugnier, J. Dumas, Sol-gel manufacturing of thick multilayers applied to Bragg reflectors and microcavities, Thin Solid Films 416 (2002) 242].
Porous Silicon (pSi) Multilayers Obtained by Electrochemical Dissolution, Alternating Layers of Different Porosity
The aforementioned characteristics make these materials excellent candidates for chemical [V. Tones-Costa, F. Agulló-Rueda, R. J. Martín-Palma, J. M. Martínez-Duart, Porous silicon optical devices for sensing applications, Optical Materials 27 (2005) 1084. T. Gao, J. Gao, and M. J. Sailor, Tuning the Response and Stability of Thin Film Mesoporous Silicon Vapor Sensors by Surface Modification, Langmuir 18 (25) (2002) 9953. Snow, P. A., Squire, E. K.; Russell, P. S. J.; Canham, L. T., Vapor sensing using the optical properties of porous silicon Bragg mirrors, J. Appl. Phys. 86 (1999) 1781] and biochemical sensors [V. S.-Y. Lin, K. Motesharei, K.-P. S. Dancil, M. J. Sailor, M. R. Ghadiri, Science 278 (1997) 840].
A large number of layers without the structural integrity problems of multilayer films obtained by sol-gel in the form of BRs can be obtained, and the thickness and porosity of each layer can be controlled in a very precise manner. The main problem of these materials is their long-term altered stability. The application of pSi BRs in air or aqueous media generates oxide on the surface in just a few hours, due to which they must be chemically modified to increase their oxidation resistance.
Multilayers of Laminae with Ordered Mesopores
This type of multilayer is manufactured by the alternating deposition, using spin-coating (S. Y. Choi, M. Mamak, G. von Freymann, N. Chopra, G. A. Ozin, Mesoporous Bragg Stack Color Tunable Sensors, Nano Letters 6 (2006) 2456) or dip-coating techniques (M. C. Fuertes, G. Soler-Illia, H. Míguez, Spanish patent with application number: 200602405), of laminae with ordered mesopores which are obtained using a template or organic mould combined with the compounds that give rise to the inorganic phase in the precursor solution which is deposited to form each layer. The porosity of these layers allows modification of their optical response by the infiltration of liquids. The possibility of functionalizing the mesopore walls in turn allows this response to be selective to a specific type or group of compounds.
Multilayers of Colloidal Particles
There is a reference in the scientific literature [I. M. Thomas, Single layer TiO2 and multilayer TiO2—SiO2 optical coatings prepared from colloidal suspensions, Applied Optics 26 (1987) 4688] to a paper wherein multilayers of alternating colloidal TiO2 particles are claimed to have been achieved, with sizes comprised between 10 nm and 20 nm, and SiO2 particles 10 nm in size. The technique used is spin-coating. In this paper, however, the microstructure of the material obtained is neither characterized nor described, nor is its mesoporosity demonstrated, by providing only an optical reflectance measure where a maximum can be observed. The applications proposed in this paper are focused on optical coatings with high heating resistance when irradiated by a high-power laser.